Conventionally, oils and fats such as oil or lard are mixed and contained in water discharged from, for example, a general house, a commercial cooking place, or a building, or water discharged from a conduit for a sewage or waste liquid treatment facility of public establishment. Such discharged water is the cause of clogging of a sewerage pipe by fixation of the oil, or the cause of an occurrence of an odor. In addition, there are also problems in that the function of the public sewerage facility is hindered or an oil clot (white solid matter) from a sewerage facility is spilled to a harbor after heavy rain, for example. Thus, in each district, measures in that restaurant companies are caused to install a gathering machine which separates and collects oils and fats in discharged water, and thus oils and fats are not spilled to the sewerage are also performed.
Treatment of separating an oil-water mixed liquid into oil and water is performed as waste liquid treatment in, for example, food manufacturing, fiber treatment, mechanical processing, and petroleum refining, and as an oil collection work performed in a case where an oil is spilled into a river, the sea, and the like due to, for example, an accident.
In addition, for example, when crude oil is minded, a method in which the seawater is injected to an oil layer of a stratum, and pressure of non-aqueous oil is increased, and thus an output is ensured is generally performed. “Water accompanying in the oil field” which is water used in such minding of crude oil contains a large amount of non-aqueous oil. Thus, treatment of removing the non-aqueous oil is performed, and then the non-aqueous oil is scrapped. However, because the non-aqueous oil is the cause of contaminating the ocean, lakes and marshes, and the like, recently, restrictions for the content of the non-aqueous oil in discharged water is reinforced. In a country or a district in which the restrictions are strongest, the content of the non-aqueous oil is required to be less than 5 mg/L.
As the conventional oil-water separation method, for example, the following methods are known: separation by using a flocculant; adhering and separation; centrifugation; pressure floatation separation; an electrolysis floatation method; coarsely granulation and separation by using a coalescer (for example, see Patent Document 1); and separation by microbial degradation.
In a case of a separation method using a flocculant, there is a problem in that expenses are continuously required, and treatment of filtered aggutinates also takes much labor and cost. A case by a machine such as a centrifuge and a case by pressure floatation separation may be effective for treating a large amount or for large-size utilities. However, the above cases have a problem in that it is difficult to be provided in a limited space. In the electrolysis floatation method, there is a problem in that complex control, for example, changing an applied power in accordance with electrical conductivity and the treated amount of a treatment liquid is required for stably performing oil-water separation. In the coalescer method, a filter having a network structure of ultrafine fiber is used. Thus, there is a problem in that clogging normally occurs in maintenance management. In a separation method using a microorganism, there is a problem in that it takes time, and maintenance is serious.
Water treatment by using a separation membrane which uses a porous film is performed in the related art. As oil-water separation, a reverse osmosis method, an ultrafiltration method, a precise filtration method (for example, see Patent Document 2), and the like are also known.
However, because oil and water is separated by using a hole diameter of the separation membrane in the reverse osmosis method, the ultrafiltration method, and the precise filtration method, there is a problem in that a membrane permeation flux is small. Further, in the process of performing water treatment, a separation target substance such as oil, which is provided in raw water adheres to the separation membrane, and thus fouling (clogging) occurs. A problem in that it is necessary that physical washing such as back pressure washing and air scrubbing is periodically performed occurs due to the fouling. Thus, improvement of difficulty in adhering oil (antifouling properties) or easiness of removing adhered oil (easy washing properties) is desired for the separation membrane using a porous film, in order to continuously use the separation membrane for a long term.
Various technologies are opened to the public for improving anti-fouling properties (clogging prevention properties). For example, a non-adhesiveness and hydrophilic hollow-fiber porous film is known. The non-adhesiveness and hydrophilic hollow-fiber porous film is formed from copolymer of polyolefin or olefin and halogenated olefin or polyvinylidene fluoride. In the non-adhesiveness and hydrophilic hollow-fiber porous film, a lateral chain including a neutral hydroxyl group is grafted to the surface of a pore in a hollow-fiber porous film, and a neutral hydroxyl group is provided. A hydrophilic fluorine finely-porous film and a water treatment method using the film are known. The hydrophilic fluorine finely-porous film is formed from fluoropolymer subjected to ozone treatment. Further, a polysulfone hollow-fiber membrane formed by polysulfone and blend of a polyvinyl acetal resin and a hydrophilic polymeric substance is known. Further, a method of applying a surfactant to a porous film which contains a polyvinylidene fluoride resin is known. The surfactant has a polyoxyalkylene structure, a fatty acid ester structure, and a hydroxyl group. A method (for example, see Patent Document 3) of performing hydrophilization treatment on a polyethersulfone porous film is known. In the hydrophilization treatment, discharging treatment is performed under vacuum while a gas mixture including a hexafluoropropylene gas and an oxygen gas flows.
However, even with the well-known technologies as described above, improvement of the anti-fouling properties may be insufficient. In a case where the above technology is used in, for example, oil-water separation, there is a problem in that oil easily adheres to a film surface, and this is the cause of decreasing a flow rate.